This invention relates to a novel method of forming spinel-related .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4, wherein 0.ltoreq.x.ltoreq.2, in a secondary electrochemical cell, and to the use of a compound of formula LiMnO.sub.2 which has a specific orthorhombic crystal structure, as initial active cathode material in such cells.
The impetus for this invention was the recent, great increase in demand for batteries having high energy density and low weight, such as had already been achieved with metallic lithium systems, but which are also rechargeable. Lithium electrodes do not meet this requirement over extended periods of time because even in an aprotic organic solvent the cycling stability is severely limited, and problems with cell safety arise.
An electrochemical cell in which both the anode and the cathode are lithium intercalation compounds is termed a lithium ion cell. The intercalation compound serves as a host structure for lithium ions which are either stored or released depending on the polarity of an externally applied potential. During discharge, the electromotive force reverses the forced intercalation or de-intercalation thereby producing current.
In a lithium ion cell, the positive electrode generally serves as the initial reservoir of lithium. The capacity of the cell will be limited by the amount of lithium present in the positive electrode on cell assembly. In most cases, only a proportion of the lithium present, during fabrication of the positive electrode, can be reversibly removed (i.e. cycled).
LiMnO.sub.2 is known to exist in several phases. Two phases, whose crystal structures have been well characterized, are a high temperature orthorhombic phase and a tetragonal phase. Both structures involve cubic closest packing but they differ in the arrangement of the ordering of the lithium and manganese cations. The tetragonal form is normally written as .lambda.-Li.sub.2 Mn.sub.2 O.sub.4. It has an atacamite-type structure which is often referred to as being spinel related. Another type of lithium manganate has a spinel structure of composition LiMn.sub.2 O.sub.4.